The present invention relates to power supplies for arc welding equipment and more particularly to power sources having a rectifier connectable to a.c. mains to provide a d.c. output which is connected to a smoothing capacitor arrangement and a parallely disposed inverter arrangement for converting the d.c. voltage to an a.c. voltage having a frequency considerably higher than that of the a.c. mains. The inverter arrangement is provided with controlled semiconductor elements and with a transformer providing for the galvanic separation of the input of the inverter arrangement from its output which may then be connected to a welding station consisting of a workpiece and a welding electrode.
Sources of power for welding are usually equipped with a transformer for reducing the mains voltage to a lower welding voltage. Thanks to the substantially higher frequency of the welding voltage it is often possible to make this transformer much lighter than would be necessary to accommodate the mains frequency. The expression substantially higher frequency is used in this context to denote frequencies of at least 0.5 KHz. It is particularly desirable to select frequencies which are above an audible range, i.e., higher than approximately 15 KHz. When compared to the mains frequency, the higher frequency permits a more rapid control of welding parameters such as welding current and voltage. Sources of power of this type have previously been disclosed in, for example, U.S. Pat. No. 4,159,409 and UK Patent Specification No. GB 2,046,535.
However, the use of higher frequencies also involves certain disadvantages. The controllable semiconductor elements needed for higher frequencies and for the high outputs required in welding applications are expensive and perform less well in certain respects than corresponding semiconductor elements usable at the frequency of the a.c. main. For instance, as a general rule these semiconductor elements exhibit poorer characteristics in relation to overloading by transient phenomena such as rapid variations in current and voltage. Further, such semiconductor elements are particularly expensive if the same are required to be capable of accepting voltage loads of the same level as normally present in a.c. mains. For example, for a mains voltage of 380 V, the smoothed rectified d.c. voltage normally exceeds 530 V. Thus, when dimensioning the semiconductor elements allowance must be made for, amongst other things, variations in the mains voltage and the presence of transient phenomena. Accordingly, when designing for a normal mains voltage of 380 V, the dielectric strength of the semiconductor element selected should be at least 600 V, while at least 800 V must be provided when a mains voltage of 500 V is being considered.
The relatively high cost of semiconductor elements exhibiting sufficiently high dielectric strength for use in power sources for welding equipment is significant, particularly in the case of smaller power sources which are manufactured in large numbers. Thus, in such smaller power sources, the cost of the semiconductor element represents a considerable proportion of the total price, especially where high voltages are involved. The use of cheaper components having lower dielectric strength is consequently particularly desirable. However, while continued development is likely to result in lower prices for such semiconductor elements, it is unlikely that any great effect on the relationship between the prices of semiconductor components which offer low dielectric strength and high dielectric strength will result.